1. Field of the Invention
The present invention relates to a code wheel manufacturing method, a code wheel, a rotary encoder, a rotation control unit, and an image forming apparatus.
2. Description of the Related Art
In the prior art a rotary encoder for accurately measuring the rotation speed of a rotary member is well known. The rotary encoder is used to control the rotation speed of a rotary member such as a carrier roller for a belt, a storage medium, an ink nozzle head, or a photoreceptor in an image forming apparatus such as an electrophotographic or inkjet type printer, a photocopier, a facsimile machine, a printing machine or the like. An optical rotary encoder is configured to include an encode sensor and a code wheel (encoder disc) having a ring-like code portion (modified optical track) including a uniform, radial code pattern on an outer circumference. The code wheel is mounted on the rotary member so that it concentrically rotates with the rotary member, and the rotary encoder can accurately detect the rotation speed of the rotary member according to signals read from the code pattern of a rotating code wheel with the encode sensor. Such a rotary encoder is adapted to various rotation speed detectors and rotation control units owing to its compact size and accurate detectability. For example, Japanese Unexamined Patent Application Publication No. 2005-134763 discloses a feedback control method using a rotary encoder for the rotation speed of a rotary member as a carrier roller of a belt carrier unit of an image forming apparatus.
FIG. 8 shows an example of a code wheel of a prior art rotary encoder. A code portion 201 (modulated optical track) is formed with micron-level high precision by etching or photolithography. One of the key factors to determine performance of the rotary encoder is precision of intervals between codes of a code pattern of the code portion 201. However, it is generally considered that the code portion 201 can be precisely formed by etching or photolithography easily, and the precision is maintainable.
Another factor to determine accuracy of signals from a code pattern is a degree of coincidence (axial runout) between a center position of the ring-like code portion 201 and a rotational center thereof when the code wheel is mounted on the rotary member and rotated. Unless the code portion 201 is rotated around its graphical center, the rotation of the code pattern becomes distorted. This makes it impossible for the encoder sensor to accurately measure signals from the code pattern of the code portion 201, decreasing precision of the rotary encoder measuring the rotation speed. An axial runout over 100 μm generally causes a large problem in practical use; however, it is preferable that the axial runout is to be 10 μm or less for the purpose of precise measurement.
To maintain concentricity of the code portion and the rotary member (suppress the axial runout within a tolerance), mainly, two techniques are known. One is disclosed in Japanese Unexamined Patent Application Publication No. 2006-292724 that positions of a code wheel and a rotary member are adjusted by optically measuring the center positions of a code portion and the rotary member with an optical microscope, to fix the code wheel and the rotary member. Accordingly, the code wheel and the rotary member can be securely fixed with the axial runout within a tolerance so that a displacement in the rotation axes is unlikely to occur. The maintenance of the concentricity of the two members enables stable measurement of the rotation speed. However, there is a problem in this technique that it requires a great amount of process, time and cost for the adjustment. It also requires precisive positioning equipment, taking more cost and time.
The other technique is disclosed in Japanese Unexamined Patent Application Publication No. 2006-300871 that a hole is formed in the center of a code wheel to be coaxial with a rotary member and fit (occasionally, with pressure) with the shaft of the rotary member, thereby fixing the code wheel and the rotary member with each other. This technique does not require the positioning process for the axes of the code wheel and the rotary member, however, the formation of the hole in the code wheel has to be precisely done, requiring a lot of work and time to examine or measure precision of the hole.
A rotary encoder manufacturing method based on the second technique is described with reference to FIG. 8. FIG. 8 shows a prior art code wheel having a mark 202 in a ring form whose center position coincides with that of a code portion 201 (concentric circle of the circumference of the code portion 201). The code portion 201 and the mark 202 can be easily formed at a micron level precision by photolithography, etching or else with their center positions coinciding with each other. The mark 202 has a line width in reality but the outer and inner circumferences thereof are both formed in perfect circles with a high precision with the line width taken into consideration. In the prior art rotary encoder, an error of about 10 μm in deviation (axial runout) of the center positions of the code portion 201 of the code wheel and the rotary member is tolerable so that an error in the center positions of the mark 202 and the code portion 201 is not a big problem.
A hole 203 is formed independently from the code portion 201, generally after formation of the code portion 201. It is therefore not easy to make the center positions of the two coincide with each other at a micron level precision. It is necessary to examine a degree of coincidence (concentricity) of their center positions for the code wheel having the hole 203. Note that the center position of the hole 203 may be considered to be a center of rotation axis of the rotary member since both are coincident with each other.
For examining the concentricity of the hole 203 and the code portion 201, the mark 202 as an almost concentric circle of the code portion 201 is substituted for the code portion 201 since direct examination of the hole and the code portion is difficult. First, coordinates (X, Y) of three arbitrary points 204 on the circumference of the hole 203 are measured with a microscope having a coordinate measuring device or a projector to calculate the center position of the hole 203, for example. Likewise, coordinates (X, Y) of three arbitrary points 206 on the outer or inner circumference of the mark 202 are measured with a microscope or a projector to calculate the center position of the mark 202. The number of arbitrary points can be more than three. A deviation in the calculated center positions of the hole 203 and the mark 202 represents a displacement between the hole 203 and the mark 202 and a deviation (axial runout) between the center position of the code portion 201 and a rotation center of the rotary member with the rotary shaft fitted into the code wheel. Code wheels with the axial runout being over a tolerance are determined as defective products and removed while ones with the axial runout being within the tolerance are determined as standard products.
However, such a code wheel manufacturing method still has a problem that examining code wheels for the axial runout is very difficult since accurate positional measurement of at least six points and calculation of the center positions are required, taking a lot of time and cost. Further, a special device such as a coordinate measurable microscope or projector is needed for the measurement of the center positions. However, such a device is not generally provided in places such as work sites of component manufacturing, or assembly line, development or distribution of a rotary encoder or a product incorporating the rotary encoder, so that the examination is unfeasible in such work sites. Moreover, manipulating the device as a microscope or a projector requires specific skills and it is not something everyone can easily do.
In reality, there may be many occasions where the measurement of code wheels is needed at work sites as above, for example, when a problem occurs in a rotary encoder in an assembly line or at a customer's work site, it is needed to find out the cause of the problem, at a general inspection in a component manufacturing process, or in testing for precision of a component in the course of product development. Every time a problem is found in a component, the component in question has to be brought to an inspection department or the like which owns a coordinate measurable device, and examined by a skillful operator, which takes a tremendous time and costs to acquire a measurement result.